1. Field of the Invention
The present invention relates to tin-lead eutectic solders, and, more particularly, to improving the fatigue resistance of such solders.
2. Description of Related Art
Eutectic and near-eutectic lead-tin solder alloys are used to provide solder joints in a wide variety of electronic devices. In addition to providing electrical connections, the solder joint provides a vital mechanical link between electronic devices and connectors.
During operation, many electrical devices are subjected to vibration and continual changes in temperature. Many times, the coefficient of thermal expansion of each the various materials at and around the solder joint is different. As a result, the continual changes in temperature cause the solder joint to be continually subjected to varying degrees of stress and strain. The solder joint may also undergo continual stress due to vibrations and other forces exerted against the joint.
It would be desirable to provide solder joints which are structurally strong and resist fatigue due to mechanical or thermal stress and strain. Such fatigue-resistant solder would be especially well-suited for used in electronic equipment which is subjected to extreme thermal fluctuations and mechanical load. Further, fatigue-resistant solder would be desirable for use in electronic devices where a long service life is required.
Fatigue-resistant solders are disclosed and claimed in application Ser. No. 08/015,875, filed Feb. 10, 1993, entitled "Improved Fatigue-Resistant Eutectic Solder", and assigned to the same assignee as the present application. The fatigue-resistant solders are lead--tin eutectic solders free of silver and gold and doped with less than 1 weight percent of at least one dopant selected from the group consisting of cadmium, indium, and antimony. The doped solder has improved fatigue resistance over undoped solders.
Continued investigations with these fatigue-resistant solders show that some desirable flux materials cannot be used to improve wetting and diminish the tendency of the solder to form balls. The reason for this is that the surfaces of the solder particles are enriched in the oxides of the Cd, In, and Sb dopants, and these oxides inhibit wetting of the metallic surfaces to be soldered. They also inhibit the fusing of solder particles into a single, massive solder joint.
Specifically, experiments by the present inventors on Cd- and In-containing solders show that these dopants tend to segregate at the surface of the solder and tend to oxidize. Their experiments also suggest that some less active fluxes are not active enough to chemically reduce the oxides of the cadmium and indium dopant species to their metallic states. As a consequence, when solder pastes made by mixing particles of these solder alloys into such fluxes are reflowed, the oxides of these elements are present on the surface of the solder particles and may prevent all of the solder particles from fusing into the solder joint. Instead, small balls of solder may form adjacent to the solder joint. These solder balls may become electrical shorts if they lodge between conductors that are at different electrical potentials. Furthermore, these surface oxides may hinder the molten solder from wetting the metal surfaces to be joined by lowering the surface-free energy of the solder surface, and thereby reducing the free energy difference between the wetted and non-wetted states that represents the driving force for wetting. Furthermore, unfused, oxide-coated solder particles in the solder joint are latent defects that may lead to premature failure of the solder joint.
One solution to this problem is to use more highly active solder fluxes, such as rosin-activated (RA), or fluxes containing chlorides, to break down these oxides during solder reflow. This is not always desirable, however, since these highly active fluxes are corrosive, and must be thoroughly removed from the printed wiring assemblies after soldering. Residual flux may diminish the lifetimes of printed wiring assemblies and other assemblies made with these solder fluxes. For this reason, the U.S. military and some commercial manufacturers of high reliability electronics prohibit or discourage the use of such highly active fluxes.
Thus, a need exists for improving the wetting of fatigue-resistant solders without the use of highly active fluxes or corrosive chemicals.